The methods of three-dimensional x-ray crystal structure determination will be applied to determine various aspects of metalloporphyrin stereochemistry. Metalloporphyrins to be studied include iron(II) and iron(III) complexes with a variety of axial ligands. These complexes are desgined to model and extend our understanding of a number of hemoprotein systems, particularly to resolve certain unanswered questions about hemoprotein structure and function. A spin-equilibrium iron(III) complex, which in one crystalline form undergoes a high-spin to low-spin transition, will be further characterized in a second crystalline modification which shows quantitatively different thermomagnetic behavior. Structure determinations at two different temperatures are planned. The synthesis and structural characterization of a number of unsymmetrically substituted iron porphyrinates will be undertaken. Further work exemplifying unusual spin state/geometry patterns in iron porphyrin complexes are planned. The characterization of metalloporphyrins in which the ring is oxidized or reduced will be studied to give insight into the interaction between metal and the porphyrin ligand. These species will have the metal atom in quite unusual formal oxidation states, and include both low- and high-valent iron derivatives.